1. Field of the Invention
This invention relates to a means for conveniently measuring the effectiveness of the bonding system in a boat or marine structure, for automatically providing optimum protection against galvanic corrosion of its immersed metal parts using a system of cathodic protection employing a galvanic anode or anodes, and for indicating the presence of stray currents from power sources which can cause rapid corrosion and/or endanger personnel.
2. Description of Prior Art
Most small to medium boats today are made of fiberglass reinforced polyester resin. Some are made of wood, aluminum or steel. When immersed in sea water, all of their underwater fittings, and in the case of metal hulls the hull itself, are subject to galvanic corrosion. This type of corrosion results from the use of fittings made of alloys and from the use of dissimilar metals in various parts of the boat hull. This hazard results from phenomena internal to and connected to the the boat structure and is, as to cause, unrelated to any external electrical connections or influences.
Galvanic corrosion may be a relatively slow process, but cumulatively over a period of time, it can waste away underwater parts thereby endangering the water-tight integrity of a boat hull and causing engine and other mechanical failures. This type of corrosion typically promotes deterioration and failure of parts made of alloys of copper, e.g. bronze, which always contain from 2% to 10% of zinc and in some bronzes (high tensile) as much as 19% zinc and 10% aluminum. If parts made of these typical alloys are left unprotected, the zinc and aluminum waste away, resulting in ultimate part failure. Brass, which is 67% copper and 33% zinc is extremely vulnerable to galvanic corrosion.
Stray current corrosion caused by unintentional leakage from boat and external power sources can be much more rapid and has resulted in spectacular failures in days, such as a whole propeller blade dropping off, rudder pintles failing, or an entire rudder breaking into pieces. Stray current corrosion should be frequently checked and immediately corrected.
An accepted practice to protect against galvanic corrosion of immersed metal parts is to use a piece, or pieces, of very pure zinc or an alloy of aluminum functioning as sacrificial anodes fastened to each underwater part where possible, or to use a large immersed sacrificial anode which is then electrically connected (bonded) to all underwater parts needing protection. This bonding connection is comprised of a heavy gauge conductor, terminating at the engine block and the negative side of the boat's battery system and forms the vessel's electrical ground.
This practice provides a galvanic couple through the sea water electrolyte between the zinc or aluminum as an anode and the bonded parts, which become the cathode. The current flowing to the cathode overpowers any galvanic cell formation within or between the dissimilar metals of the boat's immersed parts, thereby preventing the loss of metal from immersed boat parts. Instead, the sacrificial anodes will waste away, preventing any harm to the immersed metal parts of the boat or marine structure. The success of this practice depends on a good bonding system having virtually zero electrical resistance between all bonded parts and adequate anode area to provide the necessary protective current in any specific situation.
Some boat builders provide no sacrificial anodes, some do but do not bond all of the immersed metal parts, some provide improper bonding, some do provide a proper bonding system. In all but the first example above, which has no protection, the degree of protection is unknown and should be carefully checked at proper time intervals. Many boat owners merely replace the anodes when the boat is hauled and then hope they have enough protection, but not too much.
Devices are available for checking proper protection of underwater fittings. One commonly used device is a silver/silver chloride half cell which, when immersed in the salt water around the boat, will output a voltage with respect to the boat's bonding system (or to the individual fittings themselves) called the hull potentential, this being the potential of the bonded metal parts with respect to the reference electrode.
Experience has established an optimum range within which the best protection of the immersed fittings and structures is achieved. For a wood or fiberglass boat, the range is between 500 and 700 millivolts. Aluminum and steel-hulled boats have different ranges. Use of the silver/silver chloride half cell has certain serious disadvantages for the pleasure boat user. It is relatively expensive, but, most importantly, it cannot be continually immersed and after a few hours it becomes polarized and fails to function.
The latter drawback dictates that this electrode can be employed only on a short-term basis and cannot be utilized as a reference electrode for a system designed to continuously and automatically control the hull potential at a correct value over extended periods of time, independent of the sacrificial anode area (as long as there is enough) and the salinity of the sea water. As a result of a combination of several factors, most boat owners have no idea whether their boat has the proper protection. The disadvantage of under-protection is the very real danger of damage to underwater parts. Over-protection wastes expensive anodes and risks the destruction of any wood around through-hull fittings. Most fittings on fiberglass hulls are backed up by wood blocks and those blocks can be severely damaged by over-protection currents. Devices are available for interposing a manually operated rheostat between the protective anode and the bonded underwater parts. The sacrificial anode or anodes must be electrically separated from the bonded parts, connected together only by this rheostat. Actual hull potential at a given moment in time can be indicated by using an immersed silver/silver chloride half cell connected to a millivoltmeter, with the other side of the millivoltmeter connected to the bonded underwater parts. The hull potential can be set in the proper range by varying the rheostat resistance--provided there is ample sacrificial anode protection. The disadvantages of this system have been noted--expense and lack of continuous control.
There are a number of impressed current cathodic protection systems described in the literature and available on the market. Some of these control the potential of hull fittings. Some of these are automatic. Impressed current cathodic protection systems utilize a DC voltage to positively energize an immersed anode, with the negative side of the current source tied to the immersed fittings and structures to be protected.
For small to medium sized boats these impressed current cathodic protection systems have several disadvantages, as follows:
1. they can be quite expensive if they are designed to protect an entire boat; PA1 2. the installation can be complex and costly, requiring extensive and careful shielding of the anode(s) from the boat hull; PA1 3. they can require a significant amount of power; and PA1 4. if not properly installed, monitored and maintained, they can cause significant damage to the fittings and the hull coatings.